1. Field of the Invention
The present invention relates to a pass band flatness correction circuit for correcting a pass band flatness in a band pass filter and a repeater system, and more particularly, to a pass band flatness correction circuit for correcting the whole pass band flatness of the band pass filter and the repeater system in which a complementary band pass filter for producing a counter-ripple corresponding to a distorted pass band ripple of a main band pass filter is added in the main band pass filter.
2. Description of the Related Art
In general, in the case of a band pass filter (BPF), a loss characteristic in a pass band and an attenuation characteristic of stop band are determined by an unloaded quality factor (Qu) of a resonator constituting the filter. That is, a filter manufactured by using a resonator having a higher quality factor reveals a smaller loss value and a sharper attenuation characteristic, while a filter manufactured by using a resonator having a lower quality factor reveals that an insertion loss in a pass band increases and a stop band attenuation characteristic decreases.
Also, in the case of an ideal band pass filter designed using a Chebyshev or Butterworth function which is most widely used in designing a filter, a ripple in a pass band of the band pass filter should be constant irrespective of the number of stages of the filter. However, in the case that a band pass filter is implemented actually, as the number of stages in the filter, the stop band attenuation characteristic is enhanced by limitation of the quality factor depending upon the size of the resonator constituting the filter. However, the insertion loss in the pass band increases, and a center frequency and a flatness of the ripple is lowered at both ends of the pass band. As a result, the center portion in the pass band characteristic curve is convex.
That is, in the case that a filter is embodied using a dielectric such as ceramic and the other materials, performance of a resonator constituting a filter is limited according to properties of a dielectric material and an electrode material and size of the resonator, which causes an additional limitation of the filter loss characteristic and the pass band ripple characteristic. Further, a system having such a filter deteriorates flatness of the whole system to thereby cause lowering of the system performance.
In particular, in a repeater system, a band pass filter having an excellent stop band attenuation characteristic, that is, a multi-stage band pass filter should be employed in order to obtain an abrupt inter-band isolation characteristic. However, in the case of a band pass filter embodied using the conventional art, flatness of a ripple is lowered and then a power difference occurs between frequency channels, to accordingly lower quality of the whole repeater system.
For example, a general band pass filter shown in FIG. 1 has a constant ripple 1 in a pass band and transmits a signal. According to the characteristic of the filter, the magnitude of a transfer signal at the center frequency in the pass band may differ from that at the end of the pass band. The difference in the magnitude of the transfer signal is determined by the quality factor of the resonator constituting the band pass filter.
A method for improving increasing the size of the resonator constituting the filter to thereby heighten the quality factor of the resonator and to then improve a ripple characteristic of the filter, is being used, in order to solve the above problems by using the conventional art, at the time when a band pass filter and a repeater system using the band pass filter are manufactured. However, in the case that a system of a limited size is embodied, the size of the resonator is also limited. Thus, the above method cannot be a fundamental enhancing method.
Thus, in the case that a band pass filter manufactured using a resonator having a limited quality factor of a determined size as in a dielectric is employed in a repeater system, it is necessary to correct a lowered flatness of a pass band ripple in which a filter a flatness correction circuit is required therein.
To solve the above problems, it is an object of the present invention to provide a pass band flatness correction circuit for correcting the whole pass band flatness of the band pass filter and the repeater system in which a complementary band pass filter for producing a counter-ripple corresponding to a distorted pass band ripple of a main band pass filter is added in the main band pass filter.
It is another object of the present invention to provide a pass band flatness correction circuit for enhancing flatness of the whole repeater system and simultaneously and additionally enhancing a stop band attenuation characteristic by an increase in the number of stages in a filter, in a mobile telecommunication repeater.
To accomplish the above object of the present invention, there is provided an active pass band flatness correction circuit for correcting a pass band flatness of a main band pass filter including a first ripple of a first shape in a pass band transfer characteristic, the active pass band flatness correction circuit comprising: a complementary band pass filter having a pass band transfer characteristic producing a counter-ripple of a reverse shape to the first ripple with respect to an input signal, for flattening the whole pass band transfer characteristic of the filter system; an amplifier circuit installed between the main band pass filter and the complementary band pass filter, for amplifying the input signal into a desired gain value; and a first attenuator circuit installed in the rear end of the complementary band pass filter, and having an attenuation characteristic with respect to the input signal in order to enhance a reflective loss characteristic.
The pass band flatness correction circuit further comprises a second attenuator circuit coupled to any one of front ends of the complementary band pass filter and the amplifier circuit, and the second attenuator circuit having an attenuation characteristic with respect to the input signal in order to improve a reflection loss characteristic.
According to another aspect of the present invention, there is also provided a passive pass band flatness correction circuit for correcting a pass band flatness of a main band pass filter including a first ripple of a first shape in a pass band transfer characteristic, the passive pass band flatness correction circuit comprising: a complementary band pass filter having a pass band transfer characteristic producing a counter-ripple of a reverse shape to the first ripple with respect to an input signal, for flattening the whole pass band transfer characteristic of the main band pass filter; and first and second attenuator circuits installed in the front and rear ends of the complementary band pass filter, and having an attenuation characteristic with respect to the input signal in order to enhance a reflective loss characteristic.
It is preferable that the main band pass filter includes the first ripple of a convex shape in the pass band transfer characteristic, and the complementary band pass filter includes the counter-ripple of a concave shape in the pass band transfer characteristic. It is preferable that the complementary band pass filter is comprised of 2n (n is an integer) resonators.
As described above, in the present invention, the complementary band pass filter producing a counter-ripple corresponding to a distorted pass band ripple in the main band pass filter or a repeater, is added in the main band pass filter or the repeater, and accordingly the counter-ripple of the flatness correction circuit is offset by the ripple of the main band pass filter or the repeater, to thereby correct the whole pass band flatness of the band pass filter or the system, and simultaneously and additionally improve an attenuation characteristic of a stop band.